1. Field of the Invention
The present invention relates to a transmission system including a plurality of transmission apparatuses which use a single channel commonly, and to the transmission apparatuses used in the transmission system.
2. Description of the Related Art
Recently, in the field of data transmission, it is a trend that a single monitoring system integrally monitors different transmission systems such as PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous Digital Hierarchy) and so forth. Thereby, a scale of the monitoring system becomes larger.
As shown in FIG. 1, respective transmission apparatuses 1 through 6 are connected therebetween by not only user information transmission lines 7 but also order-wire transmission lines (OW transmission lines or meeting transmission lines) 8 for transmitting a monitoring signal and for transmitting an order-wire signal, which is a telephone-conversation signal, between the transmission apparatuses. Recently, as the scale of an information transmission system becomes larger, the scale of a network of the order-wire transmission lines becomes larger.
FIG. 2 shows an example of an arrangement of the transmission apparatus B2, transmission apparatus C3, and transmission apparatus E5. In this example, the transmission apparatus C3 and transmission apparatus E5 are branched off from an analog separation and combining portion 13 of the transmission apparatus B2. A digital audio signal from the transmission apparatus A1 is transmitted through an optical/radio transmission path 20, is input to an optical/radio transmission portion 10 (for transmitting and receiving optical/radio signals) of the transmission apparatus B2, then, is demultiplexed from another signal(s) through a multiplexing/demultiplexing portion 11 (for multiplexing and demultiplexing signals), and is input to a two-wire telephone 16 via an order-wire transmission portion 19. Thereby, a voice from the transmission apparatus A1 can be heard in the transmission apparatus B2. An analog signal from the telephone 16 of the transmission apparatus B2 is transmitted to the transmission apparatus A1 through a route reverse to the above-mentioned route. That is, an audio signal output from the telephone 16 of the transmission apparatus B2 is transmitted to, via the order-wire transmission portion 19, the multiplexing/demultiplexing portion 11 which then multiplexes the audio signal with another signal(s), and, then, the thus-multiplexed signal is transmitted from the optical/radio transmission portion 10 through the optical/radio transmission path 20. Thus, the digital audio signal is transmitted to the transmission apparatus A1.
The order-wire transmission portion 19 includes a codec portion 12 which performs coding and decoding, an analog separation and combining portion 13 which performs separation and combining of analog signals, a 2W/4W converter 14 which performs conversion between a 2-wire signal and a 4-wire signal, and a terminal DTMF (Dual Tone Multi-Frequency) sending/detecting portion 15. The analog separation and combining portion 13 has a function such that each port outputs a signal of the sum of the signals input from all the other ports, and, a signal input through each port does not return to this port. The terminal DTMF sending/detecting portion 15 performs sending and detecting of a DTMF signal for signaling. In a case of calling, the terminal DTMF sending/detecting portion 15 sends a calling signal. In a case where a calling signal is received by this transmission apparatus, the terminal DTMF sending/detecting portion 15 sends a call detection signal to a terminal (transmission apparatus) which has sent the calling signal. Thereby, a telephone conversation is performed in the order-wire system by using a single channel, it is possible to select/change a transmission apparatus with which a telephone conversation will be performed, and it is possible that a telephone conversation is performed between arbitrary terminals (transmission apparatuses). In the example shown in FIG. 2, the terminal DTMF sending/detecting portion 15 is provided inside the order-wire transmission portion 19. However, it is also possible that the terminal DTMF sending/detecting portion 15 is provided outside the order-wire transmission portion 19.
Operation of the order-wire transmission portion 19 will now be described. In a case of signal reception, a received digital signal is converted into an analog signal through the codec portion 12, and is then combined through the analog separation and combining portion 13 with the signals transmitted from the transmission apparatuses C3 and E5 in an analog state. The thus-combined signal is input to the two-wire telephone 16 via the 2W/4W converter 14. In a case of signal transmission, an analog signal output by the telephone 16 is input to the analog separating and combining portion 13 via the 2W/4W converter 14. The analog separation and combining portion 13 combines the thus-input analog signal with signals from the transmission apparatuses C3 and E5 in the analog state. The combined signal is converted into a digital signal through the codec portion 12, and is input to the optical/radio transmission portion 20 in a digital signal form.
In the related art, the following influences occur when the scale of the network increases:
(1) Problem of Plurality of Telephone Conversations
There is only a single order-wire channel, and a frequency of using of the order-wire channel increases when the scale of the network increases. As a result, in many cases, multiple telephone conversations are performed through the single order-wire channel. Thereby, crosstalk occurs.
(2) Noise Generation Due to Analog Coupling between Transmission Apparatuses
FIG. 3 illustrates generation of noise due to analog coupling between transmission apparatuses in the related art. In this example, the transmission apparatuses 32 and 34 are branched off from an analog coupler 44 of the transmission apparatus 31. FIG. 3 shows a situation where noise is generated when a telephone conversation is performed from the transmission apparatus 30 to the transmission apparatus 33. An analog signal `a` from the transmission apparatus 30 is transmitted via an analog-to-digital converter 40, a transmission path 35, a digital-to-analog converter 41, the analog coupler 44, an analog coupler 45, an analog-to-digital converter 42, a transmission path 37 and a digital-to-analog converter 43.
In this case, because a digital circuit does not connect (36) between the transmission apparatuses 31 and 32, the digital-to-analog converter 41 and the analog-to-digital converter 42 are needed. Further, through a transmission path between the analog couplers 44 and 45, noise .alpha. (external noise, internal noise of an electronic circuit and so forth) is picked up. This analog noise .alpha. is converted through the analog-to-digital converter 42. Accordingly, from the transmission apparatus 32, in addition to the signal received at the transmission apparatus 31, the noise .alpha. and quantization noise of the noise .alpha. are transmitted. As a result, the output of the digital-to-analog converter 43 includes, other than the signal `a`, quantization of the signal `a`, noise .alpha., and quantization noise of the noise .alpha.. In FIG. 3, f(a) represents a digital signal obtained when the analog signal `a` is quantized, and Nq(a) represents a quantization error generated at this time. Similarly, Nq(.alpha.) represents a quantization error generated when the noise .alpha. is quantized.